The invention relates to a method of electroslag welding, comprising the known steps of assigning respective optimum values and value ranges to the width of a slot defined by workpieces to be welded and to a voltage and current range necessary for welding, setting up the workpieces with the slot of optimum width therebetween, closing the slot laterally, guiding a welding filler wire into the slot, connecting the welding filler wire and a workpiece to respective poles of a suitable current source and carrying out welding by feeding the welding filler wire into the slot after having switched on the current source. The invention relates further to an apparatus for electroslag welding, particularly for carrying out the proposed method, comprising a current source connected by its poles to workpieces separated by a slot and to a welding filler wire and further comprising a feeder for guiding the welding filler wire into the slot. By the inventive process and apparatus of electroslag welding, an intermittent weld can be produced which does not need heat treatment for assuring good quality.
Electroslag welding is a process for obtaining intermittent welds of great thickness in one technological step or pass. While producing an intermittent weld between workpieces of great thickness, it is advantageous to use the method of electroslag welding, because in this way, it is possible to provide a good weld along a workpiece of 3 meters thickness in one technological pass, whereas the method of the more commonly used arcwelding renders this possible only by repeated passes of the welding electrodes along the workpiece. For performing electroslag welding the workpieces to be welded should be arranged at a determined distance from each other depending upon the workpiece and the planned welding conditions. In this way, a vertical slot is defined. The welding filler wire to be guided into the slot and the workpieces as well are connected to respective poles of a current source. The welding filler wire should be guided into the slot in such a manner that it has no metallic content with the workpieces during feeding in the beginning. The welding filler wire is fed in the slot as far as possible until an electric arc is established between the wire and the workpiece or a metallic bottom plate connected to the workpieces. The electric arc results in the melting of the filler wire and the bottom plate and in the production of a metallic molten pool. Hereupon, slag-forming powder is fed onto the metallic molten pool. The power melts and therefore becomes electrically conductive and chokes the electric arc. Slag-forming powder can be added as an insulating coating formed on the welding filler wire or on a tube for guiding it. The molten pool of slag prevents the electrical current from igniting an electric arc, and the molten slag ensures heat transfer and melting of the surface layer of the workpieces next to the slag so that welding occurs by the hot slag.
Through this welding process an intermittent weld of great mass which is similar to a column, is produced. Also several welding filler wires can be applied if required. The current rate per filler wire is commonly in the range of 500 to 800 A. It is preferred to use units with flat current-voltage characteristics as the current source, since they are relatively easy to adjust. In the case where such supply units are used, the current intensity can be well adjusted by regulating a feeding rate of the welding filler wire while the voltage value can be controlled in the current source. Up to the present, endeavors have been made to ensure, in a continuous way, constant values of current intensity and voltage of the electroslag welding because in this way the main features of the electroslag welding process are a uniform increase of the intermittent weld and a homogeneous characteristic to the weld.
One of the main problems of welding is the quality of the intermittent weld, that is the welded joint. The intermittent welds of great mass on one technological pass by electroslag welding are generally rigid and their impact strength and toughness are not satisfactory, therefore heat treatment or the application of other quality improving processes is needed. For example, the intermittent welds between workpieces of 30 mm thickness made of unalloyed structural steel are produced in general by current of the 500 to 600 A range and voltage of the 34 to 36 V range when using the method of electroslag welding. The impact strength of the intermittent weld formed in this way rarely exceeds the value of 40 J (joule) at room temperature (18.degree. to 20.degree. C.). At 0.degree. C. it comes generally 20 J, and at -10.degree. C. it remains under 10 J. The characteristic value of 35 J can be generally ensured only in a temperature range over room temperature. The mentioned disadvantageous features follow from the poor quality of the crystal structure of the intermittent weld.
In methods of electric arc welding, it is well known to use synchronous pulsating current and voltage during welding. Pulsating these parameters renders welding easier and permits vertical welding, the welding of thin plates, controlled zone melting of a seam and the like. The crystal structure of the intermittent weld can be improved in such a manner if required, however, pulsating is suitable for adjusting heat introduction, too.
The intermittent welds of large thickness which are produced by electric arc welding, are of good quality since they are influenced advantageously by welding in repeated lines, whereby each repetition (each new welding deposit) heat-treats the preceding line. In the method of electroslag welding, however, such a direct heat-treating effect has been unknown as for, although in view of quality of the intermittent weld, that is its low toughness, improving it is often required.
The use of a heat-treatment to improve weld qualities is high-priced and requires high energy consumption, because the welded workpieces of great mass should be reheated to a well determined temperature range. According to another well-known method the workpiece is produced of a material to which alloying components have been selected according to requirements of subsequent welding. The same relates to the welding filler wire, too. All these solutions cause an increase in cost and technological significance of the production.
Other quality improving methods have remained at the level of technological proposals. Examples of these are applying vibration during welding, irradiation by ultrasound or gamma-radiation, reheating with guided flame or by induction, or applying a magnetic field. The mentioned solutions are characterized by little efficiency and eventually by relatively high costs of performance, therefore quality improvement is usually achieved exclusively by the method of alloying and heat-treatment.